Antidepressant drugs down-regulate the norepinephrine or serotonin transporter (NET, SERT), resulting in enhanced noradrenergic or serotonergic neurotransmission and a persistent antidepressant-like effect on behavior, even when brain and plasma levels of the antidepressant are minimal;it is not clear as yet whether this is a general effect of antidepressants that interact directly with the transporters. Consistent with clinical observations, the persistent antidepressant-like behavioral effects observed following drug-induced NET or SERT down-regulation depend on enhanced noradrenergic and serotonergic neurotransmission, respectively. NET and SERT down- regulation appears to be a direct effect caused by drug-transporter interaction, likely involving increased internalization and degradation, and is not secondary to increased synaptic concentrations of norepinephrine or serotonin;also, it does not result from decreased transcription. Evidence to date from in vitro studies suggests an important role for protein kinase C (PKC)/protein phosphatase 1/2A (PP1/2A) signaling in the regulation of cell-surface expression of the monoamine transporters. Understanding mechanisms underlying NET and SERT regulation in vivo may provide a unique means to control their expression and function and suggest novel pharmacological methods for enhancing noradrenergic and serotonergic neurotransmission and producing antidepressant effects. This issue will be addressed by: 1) determining whether chronic treatment with venlafaxine or reboxetine, which appear to have some unique pharmacological properties, produces persistent antidepressant-like effects on behavior in a manner that depends on enhanced noradrenergic or serotonergic transmission;2) determining whether chronic treatment with venlafaxine or reboxetine alters NET or SERT function in a manner that increases monoaminergic neurotransmission;3) determining whether chronic treatment with PKC activators produces persistent antidepressant-like effects on behavior in a manner that depends on enhanced noradrenergic or serotonergic neurotransmission;4) determining whether chronic treatment with the PKC activators alters NET or SERT expression or function in a manner that increases monoaminergic neurotransmission;and 5) determining whether PKC or PP1/2A inhibition alters the neurochemical and behavioral effects of the NRI desipramine and the SRI sertraline. The completion of the proposed research will elucidate the role of altered NET and SERT expression and function in the behavioral effects of chronic antidepressant treatment and will further demonstrate the role of PKC-PP1/2A signaling. Understanding the functional significance of these processes in vivo and their underlying mechanisms will contribute to the understanding of the pathophysiology and pharmacotherapy of depression and may identify novel molecular targets for antidepressant drug discovery.